Microstructure And Wear Property Of Ceramics Reinforced Metal Matrix Composite Laser Cladding Coatings On Titanium Alloy

As one of the new-type structural materials since 1940s,titanium alloys have the advantages of high specific strength,good performance under medium or low temperature,and good corrosion resistance.Hence,titanium alloys have been widely used in aerospace,automotive industry,sport equipment,petrochemical industry,etc.Nevertheless,their application in wear and friction occasions is restricted due to the disadvantages of low hardness and poor wear resistance.In order to broaden the application range of titanium alloys,various surface modification techniques have been studied,such as chemical heat treatment,ion implantation,plasma spraying,micro-arc oxidation,and so on.Laser cladding technique has also been used for surface strengthening and modification of titanium alloys,owing to the high processing efficiency,small heat-affacted zone,the metallurgical bonding between the coating and substrate,and the controllable thickness.In this thesis,several composite materials systems were utilized for laser cladding on titanium alloys,including the "Ni60A+micro grade B4C(?-B4C)","Ni60A+Ni coated B4C(Ni@B4C)",and "Ni60A+nano grade B4C(n-B4C)".During the cladding process,complicated physical and chemical reactions occurred.For example,Ti reacted with B and C,forming TiB2,TiB,and TiC;Ni combined with Ti,forming Ni-Ti intermetallics;Ni reacted with B,forming Ni2B and Ni3B;and similarly,Cr2B,CrB,Cr7C3,and Cr23C6were in situ systhesized.The composite coatings reinforced with various in situ formed various phases were beneficial to the improvement of the hardness and wear property.In this study,the effects of process parameters(laser power,scanning velocity)and cladding materials composition on the macro morphology,defects,microstructure,microhardness,and wear property were investigated.The influence of micro/nano scale Nd2O3 or La2O3 on the cladding process was studied as well as the action mechanism.Besides,the formation mechanism of the various phases in the cladding coating was analyzed.Moreover,the strengthening mechanism and the wear mechanism were discussed.The cladding coatings fabricated with Ni60A+?-B4C mixed powders were mainly comprised of ?-Ni,TiB2,TiB,TiC,NiTi2,NiTi,Ni2B,Ni3B,Cr2B,CrB,Cr7C3,and Cr23C6.Under appropriate process parameters(e.g.laser power?2.0 kW),cladding coatings showing better surface morphology and without pores or cracks could be obtained.As to the Ni60A+20wt.%?-B4C cladding coatings,the microstructure became finer,and the microhardness decreased,in pace with the increasing scanning velocity from 300 mm·min-1 to 600 mm· min-1 under 3.0 kW laser power.The wear resistance of the cladding coating fabricated under 450 mm·min-l were enhanced to 8.30 times the TC4 substrate.By cladding with Ni60A+10wt.%B4C mixed powders,the specimens with 20?m B4C exhibited best wear resistance(5.36 times the TC4 substrate).Too much ?-B4C(?20wt.%)led to pore formation and thus restricted the further improvement of wear resistance.The rare earth oxides Nd2O3 or La2O3 were found beneficial to the refinement of the coating microstructure and further enhancing the wear resistance.However,the phase constituent was not affected obviously.Nano scale rare earth oxides showed better effect than that of micro size.In comparison,nano scale La2O3(n-La2O3)did the best.However,the addition amount of n-La2O3 should be strictly controlled.Too much n-La2O3 addition was found detrimental to the properties,owing to the high dilution rate;worse still,n-La2O3 tended to aggregate in the cladding coatings and reduced the modification effect.In this paper,the cladding coating with 1.0wt.%n-La2O3 showed the lowest wear mass loss(0.0018 g).The wear resistance was enhanced by a factor of 7.44 compared with the substrate.Similar with that of Ni60A+?-B4C material system,the scanning velocity showed minor impact on the phase constituent of the Ni60A+20wt.%Ni@B4C cladding coatings.With the increase of the scanning velocity,the microhardness and wear resistance increased(9.85?14.22 times the TC4 substrate).Interestingly,the coatings obtained with Ni60A+15wt.%Ni@B4C mixed powder under 2.0 kW/450 mm·min-1 process parameters showed dense microstructure and superior wear resistance,despite of the few pores in the surface layer.The low friction coefficient(?0.38)was ascribed to the graphite formed in the coating.As to the Ni60A+n-B4C material system,coating showing excellent wear property was fabricated with slight amount of n-B4C addition under low laser power.The Ni60A+5wt.%n-B4C coating prepared under 1.0 kW laser power and 450 mm·min-1 scanning velocity showed excellent wear resistance,which was 43.67 times the TC4 substrate.Both aggregate and spherical graphite were picked out in the coatings fabricated under the condition of 1.0 kW/450 mm·min-1(Ni60A+5wt.%n-B4C)and 2.0 kW/450 mm·min-1(Ni60A+10wt%n-B4C).The antifriction effect of graphite was remarkable.It should be noted that the coatings where graphite appeared were both manufactured under lower laser specific energy.Under higher laser power or slower laser scanning velocity,more Ti atoms were diluted into the molten pool and reacted with C,forming TiC,which was not beneficial to the formation of graphite.In this thesis,various materials systems were utilized for laser cladding on TC4 titanium alloy.Cladding coatings reinforced with multiple phases were obtained and the effects of materials composition and process parameters on the microstrucre and wear property were studied.The formation mechanism of the in situ formed phases and the wear mechanism were discussed.The whole study was expected to lay foundation for the application of the laser cladding technique in relevant industries.